The present invention relates to a heterojunction bipolar integrated circuit which uses a heterojunction bipolar transistor with a heterojunction between its emitter and base regions to constitute an unsaturated logic circuit.
It is well known that heterojunction bipolar transistors, having an emitter region formed from a material with a wider bandgap than that of the base region, offer numerous advantages over homojunction bipolar transistors, which have a homojunction between their emitter and base regions. These advantages can be summarized as follows.
(1) The emitter injection efficiency can be increased by making use of the difference in bandgaps, even if the ratio between impurity concentrations of the emitter and base regions is small.
(2) Base resistance can be reduced, since the base impurity concentration can be set to a high value as a result of (1) above.
(3) Emitter junction capacitance can be reduced since the impurity concentration of the emitter region can be set to a low value.
Heterojunction bipolar transistors thus offer superior high frequency and switching characteristics over homojunction bipolar transistors, and are particularly promising for use in microwave and high-speed logic circuit transistors.
FIG. 1 is a simple sectional view of the structure of a heterojunction bipolar transistor used in a conventional logic circuit. The transistor has the structure illustrated in order to facilitate extraction of the base electrode. In this extracted base electrode, there is a parasitic diode like the one shown in region b. The area of the junction between the base and collector regions increases due to the presence of this parasitic diode, and hence the junction capacitance between the base and collector regions also increases. Since the product of junction capacitance, between the base and collector regions, and load resistance, are the main factors determining the switching speed, an increased junction capacitance between the base and collector regions means degraded high-speed characteristics for the logic circuit composed of this type of transistor.
In logic circuits of this conventional structure, then, there is a parasitic external base region (region b) on the collector region side of the transistor. As a result, the capacitance on the collector region side is several times greater than the junction capacitance between the base and collector regions in an intrinsic transistor, thus negating much of the advantage offered by the good, high-speed characteristics of heterojunction bipolar transistors.
In order to resolve this problem, H. Kroemer describes a logic circuit using inverted structure (C-top) hetero-bipolar transistors in his article, "Heterostructure Bipolar Transistors and Integrated Circuits", Proc. IEEE, Vol. 70, 1982, pp. 13-25. In the heterojunction bipolar transistor described, however, an emitter layer is used in common by each transistor. As a result of the specific resistance of this emitter layer, differences appear in the voltage applied to each transistor, i.e., voltage V.sub.BE applied between the base and emitter, and uniform operation of the respective transistors is compromised.